Stereomicroscopes equipped with a monoscopic or conditionally stereoscopic co-observer tube (assistant's tube) are well-known and have meanwhile become indispensable in ophthalmic surgery and neurosurgery. However, depending on the surgeon's requirements or on the space available or on the site of the patient's surgery, it is desirable for the assistant to stand either at the left-hand side or at the right-hand side of the microscope. The state of the art pertaining to suitable monoscopic or conditionally stereoscopic assistant's tubes is that an optical beam splitter is employed to laterally divert an assistant's beam path from the left-hand or right-hand main beam path. With the existing solutions, only under certain limiting preconditions can the wish be fulfilled of being able to position the assistant to the left-hand or to the right-hand side as desired.
If the stereomicroscope is configured so as to divert an assistant's beam path from a main beam path on only one side, then an appropriate stereomicroscope with a left-hand or right-hand side assistant's tube would have to be selected. However, the latter selection option between two different stereomicroscopes is often not available so that in such a case, this wish could not be fulfilled at all, or else only by turning to a more costly duplicate solution.
If the stereomicroscope can divert an assistant's beam path from the left-hand as well as from the right-hand side main beam path but, at the same time, is only equipped with one assistant's tube, then said tube would have to be removed from one side and mounted onto the other side; see, for example, Leica's brochure titled “The Leica MS1—Surgical Microscope System”, Document No.: English 10 M1 500 Oen/A, II.2002, February 2002. Drawbacks here are not only the mounting itself but also the risk of sterility problems.
If the stereomicroscope is equipped with an assistant's tube on both sides, then the wish of being able to position the assistant either on the right-hand or on the left-hand side as desired could, of course, be optimally fulfilled. However, due to weight and cost considerations, fitting a stereomicroscope with two fixed assistant's tubes instead of with one swinging tube would not be advantageous either.
The disadvantage of having to remove the assistant's tube from one side and having to subsequently mount it on the other side or else the disadvantage of having to install two assistant's tubes on both sides can be overcome by solutions that entail a swinging capability in a horizontal plane. The term horizontal plane in the case at hand refers to a plane that runs at least approximately perpendicular to the main stereo beam paths. These solutions, however, all fall within the realm of complex measures to provide stereoscopic viewing for assistants. They are only feasible in those cases where the stereomicroscope is not only equipped with two main stereo beam paths but also with two additional assistant's main beam paths that run in parallel, in other words, equipped with a four-beam microscope set-up.
However, in addition to the technical complexity, this solution entailing a swinging capability still has the disadvantage that the surgeon's tube is in the way and the assistant's tube can only be swung around once the surgeon's tube has been removed or at least folded away.
Another drawback is that any data that is reflected into one of the two main stereo beam paths also has to be correspondingly reflected into one of the two assistant's beam paths.
Also in the case of monoscopic and conditionally stereoscopic solutions, when it comes to reflecting in data for the assistant, it remains disadvantageous that such data can only be reflected into the main stereo beam path from which the assistant's beam path is also diverted. With these solutions, when the position of the assistant is changed from one side to the other, this is likewise always associated with a change of the main stereo beam path. However, in order to continue to provide the assistant with the reflected-in data—at times, it is precisely the assistant who is supposed to be responsible for observing this data—the surgeon would consequently have to change the reflecting in of the data from one main stereo beam path to the other one. This situation, however, might not be desirable for the surgeon since she/he no longer has the option of providing her/his dominant eye with the reflected-in data or of leaving the dominant eye free. Moreover, the surgical microscope would then have to be fitted with a means to change the reflecting in from one main stereo beam path to the other one, or else the means for reflecting in would have to be removed so that it could then be mounted onto the desired side.